1. Field of the Invention
The present invention relates to a motor-driven compressor which is driven by a driving device connected to a low-voltage power source and through which a relatively large electric current flows.
2. Description of the Related Art
Conventional electric cars or hybrid cars are generally provided with a plurality of 12-volt batteries, and a motor-driven compressor for an air conditioner mounted therein is supplied with, for example, 200 to 300 volts of electricity.
FIG. 1 depicts a motor-driven compressor 2 for an air conditioner mounted in a conventional electric car. The compressor 2 has a shell that is made up of a generally cylindrical shell 4, a lower shell 6 joined to one end of the generally cylindrical shell 4, and an upper shell 8 joined to the other end of the generally cylindrical shell 4. The shell accommodates an electric motor 10 and a compression mechanism 12 driven by the electric motor 10. The electric motor 10 includes a stator 10a secured to the generally cylindrical shell 4 and a rotor 10b secured to a crank shaft 14 constituting the compression mechanism 12.
An electric power from a power source (not shown) is supplied to the motor-driven compressor 2 via a driving device (not shown), a wiring system (not shown), and the like. In applications where the voltage of the power source ranges from 200 to 300 volts, an electric current of only 6 to 7 amperes flows through electric wires, pins of a terminal 16 secured to the upper shell 8 of the motor-driven compressor 2, and lead wires 18 connected to the pins. Accordingly, the diameter of the electric wires is relatively small and, for example, 2-mm.sup.2 electric wires are generally used.
In some motor-driven compressors, an electric current larger than that in the compressors referred to above flows. If an electric current of, for example, 15 to 20 amperes flows in the compressors, wiring must be conducted using about 3-mm.sup.2 electric wires. In that case, a terminal larger than the terminal 16 shown in FIG. 1 is mounted to the upper shell 8.
Recently, a study of electric cars or hybrid cars for practical use is being made at a high pace, and from the viewpoint of safety, there is a tendency to use a 36-volt power source, for example, by connecting three 12-volt batteries in series.
However, if a motor-driven compressor is supplied with 36 bolts of electricity, a large electric current of about 60 to 80 amperes flows through the terminal and the lead wires of the motor-driven compressor via the wiring system. Accordingly, it is necessary for the electric wires to have an increased diameter to allow the electric current of 60 to 80 amperes to flow therethrough. The increase in diameter hardens the electric wires and reduces the degree of freedom of wiring, thus making the wiring very difficult.
Returning to FIG. 1, the lead wires 18 of stator windings have respective tab receptacles 19 connected thereto, while the terminal 16 secured to the upper shell 8 has pins and tabs welded to respective pins. In assembling the motor-driven compressor 2, the tab receptacles 19 of the lead wires 18 are connected to the tabs of the terminal 16. However, because the internal space of the upper shell 8 is extremely limited, if the diameter of the lead wires 18 is large, it is very difficult to connect the tab receptacles 19 to the tabs of the terminal 16. Furthermore, if the diameter of the lead wires 18 is large, the heat radiating properties at the connecting portions decrease.
In addition, where a motor-driven compressor is installed in an automotive vehicle, it is necessary to take the resistance to vibration, the resistance to impact, and the like into due consideration. In particular, a hermetic terminal for connection with an external power source requires good hermetic properties or resistance to thermal shock, and also requires a reduction in contact resistance in order to maintain the current-carrying ability in view of the use of a low-voltage and large-current power source.
Conventionally, as shown in FIGS. 2A and 2B, the terminal 16 secured to the upper shell 8 is connected to the external power source by press-fitting tab receptacles 22 connected to external electric wires to external tabs 20 secured to the terminal 16. However, a mere press-fitting increases the contact resistance, and if the diameter of the electric wires is large, there arises a problem in that the mutually connected elements are easily disconnected from each other by vibration.
Also, because the wiring of the large-diameter lead wires is difficult, there is a danger of the lead wires being brought into contact with the shell in the vicinity of the terminal, giving rise to a leakage of electricity.